System Monitor does not launch

As suggested from comments:

snap remove gnome-system-monitor
sudo apt install gnome-system-monitor

worked for me.

I had the same issue. The gnome-system-monitor is now available as a snap and it appears an update either broke something or didn't change everything properly (I don't really know why it stopped working).

A solution is to reinstall the snap (using snap, not apt) with:

snap remove gnome-system-monitor
snap install gnome-system-monitor

Doing this will install/connect the System Monitor properly and it will work once again from both icon and terminal.